Optical fibers form some of the main lines through which telecommunications data is connected all over the world. An optical fiber typically includes a core region surrounded concentrically by a cladding. Some fiber designs, known as “double cladding” designs, surround the core region with an inner cladding, which is in turn surrounded by an outer cladding. The outer cladding likewise is surrounded by an external medium.
The parameters in such double cladding waveguide designs usually are:
Dependent parametersMFDoperational mode field diameterλoperational wavelengthλcsecond mode cutoff wavelengthIndependent parametersrcocore radius generated from a single effective(refractive index) step approximationof the core regionricinner cladding radiusWinner cladding width (ric − rco) generated froma single effective step approximation of theinner cladding regionrocouter cladding radiusncocore refractive index generated from a singleeffective step approximation of the core regionnicinner cladding refractive index generated froma single step approximation of the innercladding regionnocouter cladding refractive indexnextexternal medium refractive index+Δ= (nco − noc)/noc−Δ= (nic − noc)/nocΔTot= |(+Δ)| + |(−Δ)|
As a light signal travels in an optical fiber the signal is attenuated, due to both material effects and waveguide effects. Waveguide effects include two categories of optical bending loss, microbending and macrobending losses.
In the early days of the fiber optic telecommunications industry, the first large-scale commercial systems were designed to operate at an operational wavelength λ of about 1300 nm, because that is a region of relatively low optical absorption loss and very low chromatic dispersion for silica fibers. Technology was developed for making optical detectors and semiconductor laser optical sources that would operate in that 1300 nm wavelength range. Thousands of miles of buried and undersea cables containing optical fibers designed for operation at 1300 nm were installed.
However, it was known that the intrinsic optical absorption losses in silica fibers were even lower at 1550 nm. This lower loss would be a great benefit in long haul telecommunications lines, because it would reduce the number of remotely powered buried or undersea repeater stations required to amplify and boost the signal along the optical path. Eventually, optical sources and detectors were developed which would operate at 1550 nm, and fiber systems based on this operational wavelength began to be installed.
Typically, one of the largest costs in establishing a fiber optic system is burying or installing the cable. In anticipation of the coming switch to 1550 nm systems, fiber suppliers began making telecommunications optical fibers that could operate at either 1300 nm or 1550 nm, such as Corning SMF-28™ single mode fiber. This fiber has a typical core diameter of 8.2 micrometers and a MFD of about 9-10 microns in the dual-band window from 1300 nm to 1550 nm. This fiber is known as a “matched clad” design with an effective step index core having a (normalized or relative) core refractive index above the outer cladding (+Δ=(nco−noc)/noc) of about 0.0035.
The first fiber optic telecommunication systems were limited to “long haul” applications from one telephone company central office to another. The recent trend had been to extend fiber optics outward from the central offices, providing “fiber to the campus” and “fiber to the desktop” in commercial buildings, and “fiber to the neighborhood” and eventually “fiber to the home” in residential areas. One example of the implementation of this trend is the Volition™ VF-45 fiber optic connector and premise “wiring” system, manufactured by 3M Company, of St. Paul, Minn., as shown in several patents, including U.S. Pat. No. 5,757,997. The connector design for this system relies on the spring force of a bent bare optical fiber end to provide engagement force and positive alignment between two optical fibers.
To minimize optical losses in connectors such as these, it is important that both fibers are designed to operate at the same wavelength and to have approximately the same mode field diameter (MFD) at that wavelength. For such connectors, it is not practical to adjust the MFD of the two fiber ends by high temperature diffusion of core dopants, as can be done when fusion splicing two optical fibers for long haul cables (see, e.g., EP 1094346 A1).
Long haul telecommunications fibers are typically kept relatively straight in large multi-fiber cables, and are thus protected from macrobending losses of light due to exceeding the critical bend radius of the fiber design (typically in the range of 25 mm to 12.5 mm). For fiber optic systems installed within commercial or residential buildings, which may include small single or duplex fiber optic cables, it would be highly desirable for the fiber to tolerate (both optically and mechanically) smaller radius bends, both for routing within walls and for jumper cables which may connect a fiber optic wall outlet to a computer or other piece of equipment. Also, the induced bend in optical fiber ends used in the Volition™ VF-45 fiber optic connector can be a source of optical loss when standard single mode telecommunications fibers are used.
As discussed above, two categories of optical bending loss are microbending and macrobending losses. Macrobending loss occurs when a length of fiber is bent into a curve such that some light is radiated out of the core into the cladding of the fiber and lost. Microbending losses result from concentrated pressure or stresses exerted on the surface of the fiber. Microbending loss occurs when the fiber is exposed to localized pressures and stress points as, for example, if the fiber is pressed against a rough textured surface (such as sandpaper). When the outer surface of the fiber is pressed against the raised points, a coating that is too hard may transfer these stresses to the core, causing scattering losses. Microbend losses are usually negligible for short lengths of fiber.
Such stresses may be reduced by providing a relatively soft, low-modulus inner coating on the surface of the glass fiber. However, usually such coatings are removed from the fiber end in order to accurately align a single mode fiber with another fiber in a connector. The stripped fiber ends are then susceptible to breakage from abrasion and moisture.
One solution to this problem is a fiber having a glass core, glass cladding, polymer cladding (GGP fiber) construction, as described in U.S. Pat. No. RE 36146, which is hereby incorporated by reference. In the present application, “GGP” coatings are defined as any of the coating materials claimed in commonly-owned U.S. Pat. Nos. 5,381,504 or RE 36,146; and U.S. patent application Ser. No. 09/973,635 (“Small Diameter, High Strength Optical Fiber”); U.S. patent application Ser. No. 09/721,397, “Optical Fiber With Improved Strength In High Humidity/High Temperature Environments”; U.S. provisional application No. 60/167,359, filed Nov. 23, 1999; and in Toray Industries, Inc., U.S. Pat. No. 5,644,670; or Showa Electric Wire & Cable Co., Ltd., U.S. Pat. No. 6,269,210 B1 (all of which are hereby incorporated by reference).
These coating materials typically have a Shore D hardness of about 55 or more, or a Young's Modulus of from 50 kg/mm2 to 250 kg/mm2 at room temperature, and they adhere tightly to the outermost glass surface of the optical fiber. They are exemplarily applied to an optical fiber such that their outer surface is sufficiently concentric with the core of the optical fiber that when a GGP coated fiber is placed in a typical fiber optic mechanical connector and optically connected to a second fiber, the optical loss is not significantly greater than for a similar connection using an uncoated fiber having the same outer diameter as the GGP-coated optical fiber. “GGP3” coatings are defined to include the GGP 3.1 and GGP 3.2 coating formulations disclosed in commonly-owned U.S. patent application Ser. No. 09/721,397, “Optical Fiber With Improved Strength In High Humidity/High Temperature Environments”, based on U.S. provisional application No. 60/167,359, filed Nov. 23, 1999. These materials are generally GGP coatings according to the definition above that are UV-curable compositions cured with a photoinitiator such as an iodonium methide salt that does not hydrolyze to release HF or Fluoride ion. GGP 3.2M coatings are defined as GGP3 coatings according to formulation GGP 3.2 as disclosed in U.S. patent application Ser. No. 09/721,397, further including an iodonium methide photoinitiator.
In a GGP fiber, the glass portion of the optical fiber is smaller than the standard 125 micrometer outside diameter, and an adherent, very concentric, and relatively hard polymer layer is added to bring the fiber diameter up to the standard 125 micrometer diameter while maintaining concentricity for connectorization. The construction is cabled within a low-modulus coating to minimize microbending losses, but when the low modulus coating is stripped off for connectorization the outer glass surface of the fiber is not exposed or damaged.
GGP coatings also provide protection for the glass surface from scratches and the moisture induced reduction in mechanical strength. A current fiber used in a “Volition™” single mode product is designed to interconnect with Corning's SMF-28 product, i.e., it has the same 2nd mode cutoff characteristic (<1260 nm), the same mode field diameters (9.2 microns @ 1300 nm and 10.4 microns at 1550 nm) and similar attenuation (<0.55 dB/km). The primary difference is that this “Volition™” fiber has a 100 micron glass diameter and three coatings including a “permanent” primary coating that results in a stripped fiber diameter of 125 microns, for fitting into standard connector ferrules and mating to standard fibers. The SMF-28 fiber has two strippable coatings over a 125 micron glass diameter. Once these non-permanent coatings on the SMF-28 fiber are removed, the outer glass fiber surface is vulnerable to the degrading effects of water and mechanical abrasion while the “Volition™” fiber remains protected by its “permanent” primary coating. However, SMF-28 fiber was designed for ultra low attenuation to minimize the need for repeaters/amplifiers in long haul telecommunications networks. A limitation imposed by matching to SMF-28 is the resulting poor bend performance inherent in the high MFD for the matched clad SMF-28 design.
Even for shorter applications where low attenuation is not a fundamental driver, the SMF 28 design places an undesirable lower limit on the bend tolerance of the fiber at the longer wavelengths—about a 1″ minimum diameter. Although a matched clad index fiber that is mode-matched to SMF-28 may provide reasonably low losses in a tight bend application such as presented by the VF45 connector, it is limited to a single wavelength—either 1300 or 1550 nm—and must have a very carefully controlled 2nd mode cutoff wavelength to provide the necessary tight modal confinement. SMF-28 and the discussed Volition™ fiber provide adequate bend tolerance at 1300 nm, but not at 1550 nm.
While a separate matched clad fiber design that is mode matched to SMF-28 solely at the 1550 nm band having a satisfactory bend loss is possible, it is less desirable from a manufacturing perspective and provides less flexibility for future changes/upgrading.
Among the optical fiber applications with the most severe bending loss requirements have been the fiber optic guided missile (FOG-M) and tethered weapons applications for the military. Here, the optical fiber that carries the target imaging data back to the operator, and also carries guidance signals to the missile, is stored on a small spool or bobbin. In addition to the bends in the many turns of fiber stored on the spool, when the missile is launched there is an extreme bend at the payoff point where the fiber attached to the missile is leaving the spool. Designs for fibers used in tethered weapon applications have concentrated on keeping the light signals very tightly confined in the fiber core, by designing fibers with small MFD (˜4-7 micrometers at 1550 nm). Some designs include a depressed refractive index well around the core (so called “W” fibers) that provide for a broader range of operating wavelengths. The high matched clad index design may also provide reasonable bend tolerance if designed to operate at a single wavelength. Examples of depressed well, small MFD fibers are described in U.S. Pat. Nos. 4,838,643, and 5,032,001.
Although these fibers meet the requirements for low bend loss, their small MFDs make them unsuitable for connectorization to the low cost, large (>8.0 microns) MFD telecom fibers. Dual wavelength versions of these fibers have the smallest MFDs and therefore the largest MFD mismatches and associated connector losses making them unsuitable for the intended application of the inventive fiber. These fibers can only be fusion spliced or thermally treated to eliminate the MFD mismatch, which is not an option in the multiple plug-in/disconnect applications.
In general, fiber designs with smaller MFDs have higher NAs at a given wavelength, since both indicate a more tightly confined optical mode, which will be less affected by macrobending or other external influences. The relationship between MFD, macrobending loss, and second mode cutoff wavelength is discussed in U.S. Pat. Nos. 5,608,832 and 5,278,931, and references therein.
Thus, there is a need for optical fibers for premise wiring and patch cables used for connecting equipment to the premise wiring that can operate at either 1300 nm or 1550 nm, have mode field diameters approximately matching that of telecommunications fibers such as Corning SMF-28™ single mode fiber, and can mechanically and optically tolerate prolonged bends with a bend radius less than half an inch (or 12 mm). Patch cord fibers would preferably work at either 1300 nm or 1550 nm. The local communications systems to which they will be connected, particularly if these are based on fibers such as Corning SMF-28™, could be operating at either (or even both) wavelengths. Also, 1300 nm Corning SMF-28™ systems may be upgraded to 1550 nm systems without installing new optical fiber cables, and it is undesirable to buy all new patch cords as part of the upgrade.